Large Spin Hall Angle Research Articles

Investigating the strain controlled epitaxial growth of Mn3Ge films through thickness modulation

Mn3Ge, a typical member of the Heusler family, has a high spin polarization and large spin Hall angle, making it a potential material for spintronic devices. Regarded as a topological Weyl semi-metal, Mn3Ge could be applied in topological spintronics owing to its special Fermi-arc-type surface states and various spin transport properties. In this study, we grew high-quality perpendicularly magnetized Mn3Gefilms through magnetron sputtering. X-ray diffraction (XRD) showed that hexagonal antiferromagnetic Mn3Ge was mixed with tetragonal ferromagnetic Mn3Ge. Thickness-dependent double-phase Mn3Ge films with large magnetic anisotropy and robust anomalous Hall effect (AHE) were obtained. The triangular spin structure of hexagonal Mn3Ge enhances the AHE; however, it shrinks the coercivity of the tetragonal ferromagnetic property. This manipulation of coexisting multi-phases comes from the strain of the substrate during growth, achieved by controlling the film thickness. Structural, magnetic, and transport measurements demonstrated stress modulation of the defect pinning, magnetic anisotropy, and spin transport properties. The coexistence of antiferromagnetic and ferromagnetic Mn3Ge provides the possibility of a new generation of Mn3X-based devices for applications in spin-torque memories.

Effect of stoichiometry on the spin Hall angle of the half-Heusler alloy topological semimetal YPtBi

We investigated the effect of stoichiometry on the spin Hall angle of the half-Heusler alloy topological semimetal YPtBi by changing the composition ratio r of Y/Pt from 0.5 to 1.9. We confirmed that YPtBi with high C1 ordering can be deposited by co-sputtering at r = 0.5–1.5, and a large effective spin Hall angle of 1.7 was achieved in a junction with Pt/Co/Pt at the exact stoichiometry r = 1.0. The –conductivity relationship is similar to that observed in samples with exact stoichiometry, indicating that the spin Hall effect in YPtBi is robust against the change of its stoichiometry.

Large Anomalous Unidirectional Magnetoresistance in a Single Ferromagnetic Layer

Unidirectional magnetoresistance (UMR) in a ferromagnetic bilayer due to the spin Hall effect (SHE) provides a facile means of probing in-plane magnetization to avoid complex magnetic tunnel junctions. However, the UMR signal is very weak and usually requires a lock-in amplifier for detection, even in a bilayer involving $\mathrm{Ta}$ or $\mathrm{Pt}$ with a large spin Hall angle (SHA). Here, we report a type of UMR, termed the anomalous UMR (AUMR), in a single $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$ layer without any adjacent SHE layers, where the UMR signal is about 10 times larger than that in $\mathrm{Ta}/\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$ structures and can be detected by using conventional dc multimeters in the absence of lock-in amplifiers. We further demonstrate that the extracted AUMR, by excluding thermal contributions, shows reversal signs for the $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$ and $\mathrm{Ni}\text{\ensuremath{-}}\mathrm{Fe}$ single layers with opposite SHAs, indicating that the AUMR may originate from the self-generated spin accumulation interacting with magnetization through the giant-magnetoresistance-like mechanism. These results suggest that the AUMR contributes UMR signals larger than the interfacial spin Hall UMR in the $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$-based systems, providing a convenient and reliable approach to detect in-plane magnetization for two-terminal spintronic devices.

Large intrinsic spin Hall conductivity and spin Hall angle in the nodal-line semimetals ThAl2 and ThGa2

The intrinsic spin Hall effect and topological properties of ${\mathrm{ThAl}}_{2}$ and ${\mathrm{ThGa}}_{2}$ have been investigated based on first-principles electronic structure calculations. The band structures calculated without the spin-orbital coupling (SOC) show that both ${\mathrm{ThAl}}_{2}$ and ${\mathrm{ThGa}}_{2}$ host multiple nodal lines near the Fermi level. Once the SOC is taken into account, these nodal lines are gapped and huge spin Berry curvatures can be generated. As a result, giant spin Hall conductances of ${\mathrm{ThAl}}_{2}$ and ${\mathrm{ThGa}}_{2}$ are induced around the Fermi level. Moreover, large spin Hall angles can be achieved due to the giant spin Hall conductance and the small electric conductivity. Our work indicates that ${\mathrm{ThAl}}_{2}$ and ${\mathrm{ThGa}}_{2}$ not only provide an ideal platform for exploring the interplay between spin Hall effect and topological property, but also have promising applications in spin-orbitronic devices.

Giant Spin Hall Effect and Spin-Orbit Torques in 5d Transition Metal-Aluminum Alloys from Extrinsic Scattering.

The generation of spin currents from charge currents via the spin Hall effect (SHE) is of fundamental and technological interest. Here, some of the largest SHEs yet observed via extrinsic scattering are found in a large class of binary compounds formed from a 5d element and aluminum, with a giant spin Hall angle (SHA) of ≈1 in the compound Os22 Al78 . A critical composition of the 5d element is found at which there is a structural phase boundary between poorly and highly textured crystalline material, where the SHA exhibits its largest value. Furthermore, a systematic increase is found in the spin Hall conductivity (SHC) and SHA at this critical composition as the atomic number of the 5d element is systematically increased. This clearly shows that the SHE and SHC are derived from extrinsic scattering mechanisms related to the potential mismatch between the 5d element and Al. These studies show the importance of extrinsic mechanisms derived from potential mismatch as a route to obtaining large spin Hall angles with high technological impact. Indeed, it is demonstrated that a state-of-the-art racetrack device has a several-fold increased current-induced domain wall efficiency using these materials as compared to prior-art materials.

Efficient and temperature-independent terahertz emission from CoFeB/NiCu heterostructures

Spintronic terahertz (THz) emission has been investigated in ferromagnetic (FM) metal/nonmagnetic (NM) metal, FM/semiconductor, and FM/topological insulator systems. However, THz emission in the heterojunction of FM and $3d$ transition light metal is rarely reported. Large spin Hall angles have been observed in a light metal alloy of the $3d$ transition metal NiCu, allowing for the emission of high-performance THz waves without the use of heavy metal. Here, we present a comprehensive study of THz emission from a CoFeB/NiCu heterostructure, with peak intensity of emitted THz pulses up to half that of CoFeB/Pt. The THz radiation from CoFeB/NiCu is shown to be generated by the spin-to-charge conversion in NiCu via the inverse spin Hall effect. The evolution of the THz signal of $\mathrm{CoFeB}/{\mathrm{Ni}}_{70}{\mathrm{Cu}}_{30}$ across ${\mathrm{Ni}}_{70}{\mathrm{Cu}}_{30}$ Curie temperature is also explored, and it is discovered that the THz emission signal in this sample is temperature independent and unaffected by the magnetic state of ${\mathrm{Ni}}_{70}{\mathrm{Cu}}_{30}$. Our findings highlight the great potential of light metal alloys containing $3d$ transition elements as spintronic THz emitters. Moreover, it has been demonstrated that THz emission spectroscopy is a helpful tool for investigating inverse spin Hall effect or anomalous Hall effect in ferromagnets.

Voltage Manipulation of Synthetic Antiferromagnetism in CoFeB/Ta/CoFeB Heterostructure for Spintronic Application

AbstractThe synthetic antiferromagnets (SAFs) with inserted heavy metal tantalum (Ta) are attracting an increasing interest due to the relatively large spin Hall angles, resulting in a much lower driven current for low‐power consumption, and the conduciveness to high magnetoresistance, showing more suitability for an application. Here, stable and reversible switching behavior is experimentally realized between antiferromagnetic (AFM) coupling and ferromagnetic (FM) coupling in Co40Fe40B20/Ta/Co40Fe40B20/(011) Pb(Mg1/3Nb2/3)O3‐PbTiO3 (PMN‐PT) multiferroic heterostructures after applying an external voltage, proved by vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR) measurements. The indirect interaction shows no periodical oscillation with the layer thicknesses variation of FM layer or nonmagnetic (NM) layer experimentally, which is consistent with the previous reports in theoretical calculation. This work is instructive and guiding for a better understanding of SAFs and realizing the next generation of AFM spintronic devices.

Improvement of the Effective Spin Hall Angle by Inserting an Interfacial Layer in Sputtered BiSb Topological Insulator (Bottom)/Ferromagnet With In-Plane Magnetization

The topological insulator BiSb is promising for spin-orbit torque applications thanks to its high electrical conductivity and giant spin Hall effect. Previous works have reported a large spin Hall angle for BiSb deposited on top of a ferromagnetic layer with perpendicular magnetic anisotropy. However, since BiSb has large surface roughness, obtaining a large spin Hall angle in BiSb (bottom)/ferromagnet (FM) is more challenging than in FM/BiSb (top) structures, especially when the FM layer is very thin. Here, we investigate the role of an interfacial layer on the effective spin Hall angle in BiSb (bottom)/FM with in-plane magnetization deposited by magnetron sputtering on sapphire substrates. We showed that inserting an interfacial layer with optimized thickness helps improve the effective spin Hall angle. We achieved a relatively high effective spin Hall angle of 1.7 in BiSb (bottom)/Ru 1 nm/Co 1 nm/Pt 1 nm structure.

Giant tunable spin Hall angle in sputtered Bi2Se3 controlled by an electric field

Finding an effective way to greatly tune spin Hall angle in a low power manner is of fundamental importance for tunable and energy-efficient spintronic devices. Recently, topological insulator of Bi2Se3, having a large intrinsic spin Hall angle, show great capability to generate strong current-induced spin-orbit torques. Here we demonstrate that the spin Hall angle in Bi2Se3 can be effectively tuned asymmetrically and even enhanced about 600% reversibly by applying a bipolar electric field across the piezoelectric substrate. We reveal that the enhancement of spin Hall angle originates from both the charge doping and piezoelectric strain effet on the spin Berry curvature near Fermi level in Bi2Se3. Our findings provide a platform for achieving low power consumption and tunable spintronic devices.

Ultrahigh efficient spin orbit torque magnetization switching in fully sputtered topological insulator and ferromagnet multilayers

Spin orbit torque (SOT) magnetization switching of ferromagnets with large perpendicular magnetic anisotropy has a great potential for the next generation non-volatile magnetoresistive random-access memory (MRAM). It requires a high performance pure spin current source with a large spin Hall angle and high electrical conductivity, which can be fabricated by a mass production technique. In this work, we demonstrate ultrahigh efficient and robust SOT magnetization switching in fully sputtered BiSb topological insulator and perpendicularly magnetized Co/Pt multilayers. Despite fabricated by the magnetron sputtering instead of the laboratory molecular beam epitaxy, the topological insulator layer, BiSb, shows a large spin Hall angle of θSH = 10.7 and high electrical conductivity of σ = 1.5 × 105 Ω−1 m−1. Our results demonstrate the feasibility of BiSb topological insulator for implementation of ultralow power SOT-MRAM and other SOT-based spintronic devices.

Thermally Robust Perpendicular SOT-MTJ Memory Cells With STT-Assisted Field-Free Switching

A back-end-of-line compatible 400 °C thermally robust perpendicular spin-orbit torque magnetic tunnel junction (p-SOT-MTJ) memory cell with a tunnel magnetoresistance ratio of 130% is demonstrated. It features an energy-efficient spin-transfer-torque-assisted field-free spin-orbit torque (SOT) switching and a novel interface-enhanced synthetic antiferromagnet (SAF). The optimal SAF with a Ru (9 Å) spacer sandwiched by Co/Pt multilayers has a high SAF coupling field of 2.8 kOe. The parallel magnetic coupling between the CoFeB-based reference layer and the bottom Co/Pt multilayer is enhanced by a magnet-coupling face-centered cubic textured Co/Pt (5 Å) multilayer buffer. The thermally induced Pt–Fe interdiffusion is effectively reduced by the W (3 Å) trilayers of texture-decoupling diffusion multibarrier. The Ta/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -W and TaN/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -W composite SOT channels are thick enough to be the etching stop and sustain 400 °C annealing without transforming to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> -W. Using the harmonic Hall voltage measurement, the Ta/W and TaN/W channels exhibit the large effective spin Hall angle of approximately −0.21 and −0.27, respectively. Scaling magnetic tunnel junction (MTJ) down to 30 nm size can reduce the switching time due to single-domain switching based on the micromagnetic simulation. The damping constant of ~0.018 is obtained by the ferromagnetic resonance measurement. A bigger damping constant reduces the switching time as predicted by the calibrated simulation.

Ferromagnetic Resonance of a [GeTe/Sb2Te3]6/Py Superlattice

A [GeTe/Sb2Te3] superlattice is known as a topological insulator. It shows magnetic responses such as magneto-optical effect, magneto resistance, magneto capacitance, and so on. We have reported that [GeTe/Sb2Te3] superlattice film has a large spin–orbit interaction using a spin pumping method of a [GeTe/Sb2Te3]/Py superlattice. In this paper, we demonstrate a ST-FMR (spin transfer torque ferromagnetic resonance) of the [GeTe/Sb2Te3]6/Py superlattice, compared with a W/Py bilayer. The superlattice film showed a large resonance signal with a symmetric component. The ratio of symmetric components (S) to anti-symmetric (A) components (S/A) was 1.4, which suggests that the superlattice exhibits a large spin Hall angle. The [GeTe/Sb2Te3] superlattice will be suitable as a hetero-interface material required for high performance spintronics devices in future.

Spin–orbit torque magnetization switching in a perpendicularly magnetized full Heusler alloy Co2FeSi

To optimize the writing and reading performance of magnetic random-access memory (MRAM) devices, achieving current-induced spin–orbit torque (SOT) magnetization switching in perpendicularly magnetized full Heusler alloys is vitally important. For conventional SOT-metal bilayer systems, heavy metals (HMs) with a large spin Hall angle (θSH) are generally used for generating a spin current, which is injected into the adjacent ferromagnet (FM) layer and exerts a torque on the magnetization to switch it. However, the large resistivity of generally used HMs such as β-Ta and β-W can increase the Ohmic loss. In this article, we achieve full SOT switching in Heusler alloy Co2FeSi using low-resistivity Pd as a spin current generation source. The critical switching current density is found to be 3.7 × 107 A cm−2, which is in the same order of magnitude as that required for conventional HM/FM systems even though Pd has a smaller θSH than that of generally used HMs. Using harmonic Hall measurements, the damping-like and field-like effective fields per unit current density are estimated to be 56.9 (10−7 Oe A−1 cm2) and 39.8 (10−7 Oe A−1 cm2), respectively. This high efficiency can be attributed to the excellent lattice matching between Co2FeSi and Pd (only 2% mismatch), to a slight Pd diffusion, and possibly to the additional SOTs induced by the in-plane spin component generated in the Co2FeSi layer. Our finding will advance the development of SOT-MRAM devices with both better reading and writing performance.

Low power spin–orbit torque switching in sputtered BiSb topological insulator/perpendicularly magnetized CoPt/MgO multilayers on oxidized Si substrate

Topological insulators (TIs) are promising for efficient spin current sources in spin–orbit torque (SOT) magnetoresistive random access memory (MRAM). However, TIs are usually deposited by molecular beam epitaxy on single crystalline III–V semiconductor or sapphire substrates, which are not suitable for realistic applications. Here, we studied SOT characteristics in sputtered BiSb topological insulator—Pt/Co/Pt—MgO heterostructures deposited on oxidized Si substrates, where Pt/Co/Pt trilayers have a large perpendicular magnetic anisotropy field of 4.5 kOe. We show that the BiSb layer has a large effective spin Hall angle of θSHeff = 2.4 and a high electrical conductivity of σ = 1.0 × 105 Ω−1 m−1. The magnetization can be switched by a small current density of 2.3 × 106 A cm−2 at a pulse width of 100 µs, which is 1 or 2 orders of magnitudes smaller than those in heavy metals. Our work demonstrates the high efficiency and robustness of BiSb as a spin current source in realistic SOT-MRAM.

Antiferromagnetic interlayer exchange coupling and large spin Hall effect in multilayer systems with Pt/Ir/Pt and Pt/Ir layers

We investigated Pt/Ir/Pt and Pt/Ir multilayers as candidates of nonmagnetic spacer layers in synthetic antiferromagnetic (AF) layers, which are available for the systematic study on AF spintronics. In these systems, we observed (i) AF interlayer exchange coupling in Pt/Ir/Pt and Pt/Ir nonmagnetic spacer layers sandwiched by Co layers and (ii) large spin Hall conductivity in Pt/Ir multilayer heavy-metal systems which is essential to achieve low power consumption spin-orbit torque switching. We found that total nonmagnetic spacer layer thickness $[{t}_{\mathrm{total}}={t}_{\mathrm{Pt}}(\mathrm{Pt}\phantom{\rule{0.16em}{0ex}}\mathrm{thickness})+{t}_{\mathrm{Ir}}(\mathrm{Ir}\phantom{\rule{0.16em}{0ex}}\mathrm{thickness})]$ range in which AF interlayer exchange coupling is observed is wide in Co/nonmagnetic spacer layer/Co with Pt/Ir/Pt and Pt/Ir nonmagnetic spacer layers. Moreover, the large spin Hall angle of ${\ensuremath{\theta}}_{\mathrm{SH}}=10.3%$ and low resistivity of ${\ensuremath{\rho}}_{xx}=35\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$ in Pt/Ir multilayer heavy metal are observed. These results indicate that Pt/Ir/Pt and Pt/Ir are nonmagnetic spacer layers allowing us to achieve the AF interlayer exchange coupling and generation of large spin-orbit torque via spin Hall effect in synthetic AF coupling layer system.

Giant and robust intrinsic spin Hall effects in metal dihydrides: A first-principles prediction

Topological nodal line semimetals offer an attractive platform to investigate exotic phenomena related to spintronics. Herein, we discover the existence of topological nodal-net semimetal states in metal dihydrides $M{\mathrm{H}}_{2}$ ($M=\mathrm{Hf}$, Th), a class of well-known hydrogen storing and superconducting materials. These materials host a fascinating nodal-net-like band degeneracy around the Fermi level, including triply degenerate points and multiple Dirac nodal lines (DNLs) in three mirror planes. Importantly, the inclusion of spin-orbit coupling breaks the degeneracy of DNLs, contributing to giant spin Hall conductivity values that are robust against large concentrations of charge doping. Furthermore, due to the rather small intrinsic electronic conductivities, large spin Hall angles can be achieved in $M{\mathrm{H}}_{2}$ for practice applications.

In-plane crystallographic orientations related spin-orbit torque in epitaxial Pt(111)/Co/Ta heterostructures

Current induced spin–orbit torque (SOT) in heavy metals with strong spin–orbit coupling strength has attracted considerable attention due to its potential applications in spintronic technology. Pt, as one of the mostly used heavy metals in SOT-based spintronic devices, shows large spin Hall angle (θSH) with its single phase and alloy counterparts. In this work, the in-plane crystallographic orientations related θSH of epitaxial Pt(111) layer is reported in MgO(111)/Pt(111)/Co/Ta heterostructures with strong perpendicular magnetic anisotropy. The θSH shows a quite large difference with values, respectively, around 0.083 and 0.057 when the current applied along the [11¯0] and [112¯] crystallographic directions of Pt(111) by the damping-like SOT efficiency using the harmonic Hall voltage measurement technique. The critical switching current densities also show large difference between these two orthogonal crystallographic orientations with the trend of that the larger SOT efficiency leads to the smaller critical switching current density. It independently confirms the generation of different damping-like SOT efficiency when current along [11¯0] and [112¯] directions of Pt(111). Moreover, a perpendicularly magnetized Pt/Co/Ta reference heterostructures with Pt having polycrystalline phase shows tiny variation of damping-like SOT efficiency in in-plane two orthogonal directions, which also indirectly indicates the crystallographic orientations related θSH in epitaxial Pt(111) layers. This study indicates that the θSH of epitaxial Pt is a crystallographic orientations related parameter.

Large spin Hall angle enhanced by nitrogen incorporation in Pt films

We report on the enhancement of spin Hall angle from the CoFeB/Pt interface by introducing nitrogen into the Pt thin film. Spin-torque ferromagnetic resonance measurements on the effective spin Hall angle (θSH) reveal a non-monotonic variation as a function of the amount of nitrogen gas introduced, Q in the film deposition, which peaks at θSH = 0.16 when Q is 8%. Our analysis shows that the θSH enhancement is mainly attributed to the increase in spin-dependent scattering at the interface. The effective magnetic damping decreases with increasing Q due to the reduced spin–orbit coupling. The interfacial spin transparency is also observed to show improvement after the introduction of nitrogen. Moreover, the additional damping-like torque from the interface may also lead to the enhancement of the linewidth modulation.

Inverse Spin Hall Effect enhancement in Pt layer doped by Ge

We systematic studied the spin pumping (SP) and inverse spin Hall effects (ISHE) in yttrium iron garnet Y3Fe5O12 (YIG)/PtGe bilayer using a coplanar waveguide based broadband ferromagnetic resonance setup. More efficient spin injection occurs at the YIG/PtGe interface. The spin Hall angle of Pt (4.4%) and PtGe (9%) are obtained from the metal layer thickness dependent inverse spin Hall voltage measurement. PtGe films are promising materials with large spin Hall angles for future spintronics applications.

Independence of the Inverse Spin Hall Effect with the Magnetic Phase in Thin NiCu Films.

Large spin Hall angles have been observed in 3d ferromagnets, but their origin, and especially their link with the ferromagnetic order, remain unclear. Here, we investigate the evolution of the inverse spin Hall effect of Ni_{60}Cu_{40} and Ni_{50}Cu_{50} across their Curie temperatures using spin-pumping experiments. We show that the inverse spin Hall effect in these samples is comparable to that of platinum, and that it is insensitive to the magnetic order. These results point toward a Heisenberg localized model of the transition and suggest that the large spin Hall effects in 3d ferromagnets can be independent of the magnetic phase.